Method of activating cathodes



Patented: Feb. 2, 1944 UNITED snares PATENT orncs mrrnon or AGHVATINGcarnon'rs Eugene Lemmers, Cleveland Heights, Ohio, assignor to GeneralElectric Company, a corporation of New York No Drawing; Application June26, 1943,

Serial No. 492,648

3 Claims. This invention relates to exhausting discharge devices and.activating their electrodes, whether cathodes or (in some cases)activated anodes.

When suitably carried out, my novel method or technique results in asuperior cathode, characterized by a very low cathode drop, and allowsof effecting the exhaust and activation expeditiously and with ordinarycommercial equipment, such as is commonly used for evacuating dischargelamps. This application is a continuaactivate the .alkaline-earth-metaloxide materialbecomes an incident or sequel oi the exhaust processing ofa discharge device, which includes evacuation of air'from the device,"degasslng the internal walls and parts of occluded or adsorbed air,gas, or moisture, and charging the device with any desired workingatmosphere or substance (s)such as inert rare gas like argon,

tion-in-part of my application Serial No. 470,119,

filed December 26, 1942, assigned to the assignee of this application.

To reduce the work function of cathodes of electric discharge devicesand increase their emission relative to the "cathode drop or fall involtage, activating alkaline-earth-metal-oxides are very commonly used,either in the form of surface coatings on the metal cathod s, or in theform of inclusions or impregations in the metal. 'The oxides in questioninclude those of barium, strontium, and calcium, mixtures 01'. severalor of all of these being generally preferred,-though barium andstrontium oxides are considered specially-desirable. Because of thegreat susceptibility of barium and strontium oxides to contam inatingreaction with moisture and with carbon dioxide in the atmosphere,however, the activating substances actually applied to metal cathodes inordinary practice ,are not the oxides themselves but otheralkaline-earth-metal compounds,

. which can afterward be decomposed to yield the oxides by-heating inasuitable protective environment, very commonly, in the dischargeenvelope of the lamp or other device, after the oath-' odes have beensealed into this envelope. .By such heating, furthermore, the activatingalkaneon,- xenon, etc., or mercury or other vaporizable and ionizablemetal, or both. The evacuation and degassing are effected by connectingthe device to a pumpinh or exhaust system and heating it to expeloccluded gases and vapors, which are drawn out by the pumps; and solikewise are the gases or vapors that are liberated from thealkaline-earth-metal oxide material in activating the cathode(s). The,heating of the dischargeenvelope-and the internal parts for thesepurposes is usually partly external and partly internal: i. e.,- it isefi'ected partly by having the discharge device in a. suitably heatedoven while being evacuated, and partly by passing current through theelectrodes of the device, when these electrodes are of filament type.Other types of electrodes may be heatedby high-frequency methods.

Now while. the general method of exhaustprocessing and activation justoutlined produces discharge devices which start and operate.satisfactorily in many cases, difliculty is sometimes encountered inproducing devices that are always satisfactory for their rated voltage,especially when this is a relatively low voltage. The trouble in suchcases is that a great many of the lamps "turned out prove, to beinoperable on their rated voltage, though perhaps capable of operationon somewhat higher voltages. Such high shrinkage in outputofsatisfactory lampscan, indeed, be

line-earth-metal oxide material used (whether the oxide itself withatmospheric contamination that is practically unavoidable, or. someother compound that is decomposible to oxide by heat) is activated andrendered copiously electronameliorated by greatly extending the timespent in' the. exhaustion, or by laboratory expedients such as the useof liquid air or dry ice traps in the exhaust system, or even by the useof getters to clean up gas in the devices: but these remedies areobjectionable and impracticable for commercial production of lamps-eventhough getters can be and are wide used in the manufacture of vacuum orradio tubes. This is because such getters usually form obscuringdeposits on the light-- transmitting lamp envelope, which seriouslyreduce the desired radiant output.

I have found that such difllculties in the production of dischargedevices that will operate on the desired voltage can be overcome bymodification of the technique outlined above, without necessity forresorting to any or the disadvantageous remedial measures that have beencarbonates (which I rather prefer to other materials above mentioned),initial decomposition is satisfactorily efiected by heating attemperatures of the order of 1100 C. or somewhat lower; and the ultimateactivation is satisfactorily effected at temp'eratures'of the order of.

1200 C. to 1300 C., with a preference for the higher temperature limitof this range.

As already mentioned, the decomposition of the alkaline-earth-metalcompound employed or present on the cathode(s) results in a considerableliberation of gas or vapor, which is mainly carbon dioxide whenalkaline-earth-metal carbonate is used. Hence it is desirable toinitially heat and outgas the cathode(s), at some 1100 C., while thedischarge device is connected to the exhaust system as above mentioned.But when this outgasing is done thoroughly, the subse-- quent higherheating to activate the oxide results in very little evolution of gas orvapor. After the evacuation and degassing of the discharge envelope andthe outgassing of the cathode(s), the envelope is charged with argon orother starting gas of similar character, or with mercury or the like, orboth, and is sealed off from the exhaust system; and the cathode(s) areheated to a higher temperature, preferably of the order of 1300 'C.,more or less, which results in a high degree of activation and a lowstarting and operating voltage for the discharge device. Apparently suchsmall amount of gas as is liberated from the cathodes during the higherheating-which is presumably oxygenreacts with the mercuryor other metalvaporv in the device, or with the electrodes or other heated metalinside the-discharge envelope, since it does not remain detectable inthe discharge atmosphere. The decomposition of oxide during activationmay be a reduction eifected by carbon of the nitrocellulose or othercarbonaceous binder that is used to coat the alkalineearth-metal oxidematerial on the cathode(s), or it may be a mere thermal dissociation,probably favored by the relatively low pressure in the dischargeenvelope when it occurs.

The success of the technique described in producing superior activationand voltage characteristics of the cathode(s) Y would seem to be due,

to the circumstance that in .this technique the' small amount ofallgaline-earth-metal or its lower oxide to which activation is due isnot liberated, presumably, until the discharge envelope is sealed ortipped oi! from the exhaust system,

,and hence is not exposed to react with the water-vapor present in thewhole exhaust system, but only with the much smaller amount of watervapor present in the sealed-on discharge envelope itself. In otherwords, it is to. be sup- This This seems confirmed by the fact thatifthe temperature of heating of the cathode(s) before the device issealed oil is much above 1100 C.-- say 1200 C.-'-only an occasional verylow-voltage lamp operable on its rated voltage will be obtained.

The practical application of my invention may be illustrated bydescribing its use in exhaustprocessing the low-voltage fluorescentdischarge lamps described in my U. S. application Serial No. 470,119,filed December 26, 1942, these lamrps being known commercially by thedesignation 3W RP-l2, and being capable of readily starting andoperating on a 12 volt D. C. circuit. A feature of these lamps is anarrangement of fixed cathode and anode electrodes so that while they areclosely spaced foreasy starting, their discharge gap in operation (1.e., the distance from the anode to the point of the cathode where the"cathode spot forms) is substantially longer than this spacing that iseflective in starting,

whereby a good light output is secured.-The1 arc voltage of such a lampmay be as low as 6 /4 to 8 /2 volts, and it may contain argon at apressure ranging from /2 to 8 mm., preferably 3 to 4 mm., as well as anamount of mercury such that not all of it is vaporized during operation.

The lamp cathodeomay consist of triple coils of tungsten wire such asdisclosed in U. S. Patent 2,306,925 to John O. Aicher, granted December29, 1942, coated with-an activatingmix- ,ture of barium and strontiumoxides, and

mounted in an RP=-,12A2 bulb. p

In coating the filamentary cathode coil of such a lamppa mixture ofcarbonates of the desired alkaline-earth-metals' is suspended in anitrocellulose binder andthus applied to the coil and dried on, etc., inthe usual way. After the electrodes have been mounted in the lamp bulb,the lamp undergoes anexhaust schedule on the pumps wherein the cathodeis outgassedat a temperature above that at which the carbonatesdecompose but not exceeding 1100 C.

at the maximum, so that the cathode is 'not activated during theexhaust. The preferred temperature for the outgassing is about 1050 C. v

While an oven is preferably used to degas the lamp envelope, the heatingof the filamentary cathode for outgassing is effected by connecting thecathode across 'a suitable source of current,

and is continued until no more carbon dioxide (CO2) comesoil, which maytake some 3 to 5 minutes, more or less. After sealing off from theexhaust system, the lamp undergoes a seasoning schedule in which currentis passed through the filamentary cathode to heat it to about m c, forsome two to six minutes, thus fully activating the cathode and bringingthe arc and starting (or break-down) voltages of the lamp within theproper limits-a maximum D. C. are voltage of 9, and a D. C. startingvoltage of 12, say.

What I claim as new and desireto secure by Letters Patent of the UnitedStates is:

1. vAn improvement. in exhaust and activationprocessing an electricdischarge device having cathode means comprising activatingalkalineearth-metal oxide material. and yielding gas when heated; whichimprovement comprises heating and degassing'the discharge device and itssaid cathode means, While ;concurrently withdrawing the liberated gasfrom the device into an exhaust system .in communication with V saiddevice, all at temperatures too low for effecting activatingtransformation of. alkalinecarting-metal oxide; sealingoi! device irtun'said exhaust system; and thereafter heating its said cathode means tohigher temperature, re-. sulting in activating transformation ofalkalineearth-metal oxide in said cathode.

2. An improvement in exhaust and activationprocessing an electricdischarge device having cathode means comprising alkaline-earthmetalcompound that is initially decomposed by heat to yieldalkaline-earth-metal oxide, which is in turnjurther decomposed andactivated; by higher heat; which improvement comprises heating anddegassing the discharge device, and

initially decomposing said aikaline-earth-metal compound as aforesaid,while concurrently withdrawing the liberated gas from the device into anexhaust system in communication with said device, all at temperaturestoo low for eflfeotinz the further decomposition aforesaid; andthereafter sealing oil. said device from said exhaust system and heatingits said cathode means t higher temperature, resulting in the aforesairifurther decomposition and activation.

3. An improvement in exhaust and activationprocessing an electricdischarge dwice having cathode means comprising alkaline-earth-metal 1EUGENE

